I've used the 135mm Rodagon for 4X5, and it works well. From what you say, a condenser problem is certainly possible, as discussed above. You also need to be aware that the turret places the lens further away from the negative and creates a little tunnel between the lens board and the rear element. Since 135mm is somewhat wide angle, this may be the problem if the condenser alignment doesn't cure the vignetting.
I believe the enlarger has an adjustable bellows, so this is a non-issue.
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